Currently, it is still possible to use the xtcore.js on your sites. However, you won't be able to enjoy the latest functionality available in smarttag.js because the xtcores.js tracker is no longer maintained.
In addition, the Smarttag offers a better flexibility when it comes to trigger hits (under special conditions for instance), or dynamically for One-page apps that rely on recent technologies such as AngularJS.
In the eyes of a developer, the SmartTag is a more modern and flexible tool, easier to use and understand than the xtcore.
Finally, this tracker is optimized and offer better performances, particularly thanks to its light-weight file in comparison to the xtcore.js.